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Make and model would help. How about my best guess?
OHH means OverHeated Heater caused by an error
One of the Water Temp Sensors has detected 118°F (47.8°C) In the Heater and Spa Functions are Disabled. You Must Press Any Button to Reset when Water is below 108°F (42.2°C). Check for Water Level, Flow Restrictions, Closed Valves, Air Bubbles, and Pump Prime, or Continue for a Detailed Test Procedure.

WARNING:
A Heater Overheat Error Message may indicate Excessively High Heater Temperatures which can Scald or Burn!
If the Heater is Not Cool Enough to Safely Service, allow the Heater to Cool before Continuing Testing.
A Heater Overheat Condition that generates an "OHH" Error Message is Most Likely the result of a Low Flow Condition. Therefore, "OHH" is considered to be a Flow Related Error.
Check for All of the following Conditions:
1.Low Water Level.
2.Blocked Suction Fitting.
3.Blocked Skimmer.
4.Dirty Filter.
5.Too Many Closed Jets.
6.Closed Slice Valve.
7.Heat Related Pump Not Running.
8.Heat Related Pump Not Primed.
9.Contact your Dealer or Service Organization.

DO NOT remove the protective covers from any electrical enclosure, or attempt to service any related electrical device, unless you are a qualified electrician or service professional.
As a responsible adult, always check the topside control panel display for errors and verify safe water temperature before using a spa, and before allowing children to use a spa. Safety should ALWAYS be your first consideration

Maybe that at lower pump speed the water is staying in the heat exchanger too long, and the rattle you hear is water starting to boil. The high limit sensors will sense the water temp being too high and shut down the heater. If it functions normally at high speed the issue is probably with the heater having problems with slow flow through the exchanger. You may want to consider switching to an electric heater where this would not be an issue. Just a guess without being there.

Check for water flow / water flow restrictions / and water flow blockages.

Many things create water flow reductions: Dirty filters, plugged impellers, packed with debris baskets - both skimmers and at pump, closed to partially closed valves, air suction leaks: worn shaft seal, worn orings at pump lid or pump volute body, low water level, stuck skimmer weir blade/gate in the up position & skimmer empties and air is sucked into the system which may cause the pump to lose prime. This a problem that goes un-noticed and is hard to detect even by some service technicians!

It is possibly coming from the heat exchanger. Most heaters have a copper tube style exchanger, capped at each end by plastic or metal headers. If the tubing has been subjected to any corrosive water condition such as low PH, low calcium hardness or very high cyanuric acid levels, the tubing may have deteriorated and is leaking. You may have dismantle the heater further and check for leaking.

After raising the water level in the pool I had no results. I bypassed the pressure switch and nothing happened so it was not the switch. Then I turned on the current for a few minutes, this moved something as it works now. So I conclude that the problem came from letting my water level get a little low.

Sounds as if the 'mode' for the tub is set to sense the temperature of the water. see if there is a setting on the control box to a 'timed' setting for filtering and heating instead of controlled by the temperature setting. the problem is the the control box is away from the water and when it calls for heat it is because the water is cooler there than in the hot tub. so when it calls for heat and the water starts to circulate it gets the hot water from the body of the hot tub and then shut down because it senses the 'hot water'. make sense?

An electric heat pump needs heat available to pump. If it is 30 degrees outside, heating a spa to 102 or so, may be impossible unless the heater has some type of alternative heat source.
A properly sized, gas fired heater can take care of a larger range of heating needs.
A combination heat pump and natural gas heater, probably with a controller, really makes the issue more difficult, expensive, and would require a lot of study for efficient, cost effective operation.
Bottom line: In really mild climates a heat pump might do the job. In cooler climates the gas heater rules. And where a wide range of temperatures and high heat demand is wanted by the customer, a gas heater is the only way to go. A backup heat pump may take years to payout.